In some rigid "pin" or "post" type insulators, an electrical cable is attached directly to the head of the dielectric; this implies special mechanical strength requirements for the glass to ensure against accidental breakage of the dielectric head and possible dropping of the cable.
Thus, for certain utilizations, the operating requirements are particularly strict and include resistance to sudden temperature changes on the order of at least 70.degree. C., and an ability to withstand accidental impacts.
Annealed glass dieletrics fail to meet the above-stated temperature specification and afford insufficient impact resistance. Highly thermally tempered glass dielectrics however withstand sudden temperature changes of far more than 100.degree. C. and exhibit very good impact strength due to their very great surface stresses. In fact, a glass of this type, with a thickness of about 10 to 15 mm, exhibits a substantially parabolic stress curve across its thickness: the surface compressive stresses can reach several hundreds of Megapascals and the internal tensile stresses are very nearly half the compressive stresses. Nevertheless, when such a dielectric is impacted with an energy greater than that of the prestressing in the glass, a breakage of the dielectric results.
The present invention is directed to providing a dielectric being free of the latter disadvantage, yet meeting the other requirements mentioned above.